The present invention relates to a fuel cell system with fuel cells assembled in modular manner, and having at least one channel system for the inlet, circulation and outlet of fluid operating substances.
In order to achieve a high level of efficiency when generating power, devices are provided which control or adjust pressure, temperature, humidity and/or the composition of at least one operating substance. To detect the actual values of the variables to be controlled or adjusted, sensors are used which are preferably arranged on a storage tank, on a device for producing the substances to be reacted and/or on a channel of a channel system.
German patent document DE 100 47 248 A1 discloses a typical structure of a proton exchange membrane fuel cell system (PEM fuel cell system), with a cathode electrode, an anode electrode and a matrix, which together form a membrane electrode unit (MEA). Both the cathode electrode and the anode electrode have an electrically conductive body, which serves as a support for a catalyst material. The matrix is arranged between the cathode electrode and the anode electrode and serves as a support for an electrolyte.
A plurality of fuel cells are stacked on one another with interposed separator plates. The inlet, circulation and outlet of oxidants (oxidizing agents), reductants (fuel) and coolants proceed via channel systems, which are produced using the separator plates. For each liquid or gaseous operating medium, supply collector channels, distributor channels and outlet collector channels are provided in the fuel cell stack, separated from one another by sealing means. Via at least one inlet collector channel, the cells of a stack are supplied in parallel with an oxidant fluid, a reductant fluid and a coolant. The reaction products, excess reductant and oxidant fluid and heated coolant from the cells are fed out of the stack via at least one outlet collector channel. The distributor channels form a connection between the inlet and outlet collector channels and the individual active channels of a fuel cell. The fuel cells may be connected in series to increase voltage. The stacks are closed by end plates and accommodated in a housing, the positive and negative poles being conveyed outside to a consumer unit.
In a PEM fuel cell system, in which the fuel contains hydrogen, a gas mixture of air and water vapour is supplied on the cathode side. The dew point of the gas mixture should be approximately 5° C. below the gas temperature. To achieve this at any desired operating temperatures, a device may be provided for humidity control which has constant actual value determination.
It is known to use an optical or capacitive sensor to measure actual humidity. The reliability and measuring accuracy of these sensors is impaired, however, if a liquid film forms on a sensor surface.
It is also known to determine the actual humidity level indirectly, by measuring temperature and pressure. Such measuring instruments, however, are complex, and have a low dynamic range of measurement.
In German patent document DE 101 49 333 A1, the humidity of a gas is determined with a resistance measuring structure that cooperates with a layer of carbon black. The speed of measurement is low, though, and the resistance measuring structure can be used only in a narrow channel of a fuel cell system due to its structural size.
German patent document DE 199 44 047 A1 discloses a device for measuring the concentration or density and the velocity of particles in a flowing fluid. For this purpose, an ultrasonic detector is provided on a channel wall. The sound waves issuing from an ultrasonic transmitter in multifrequency operation are reflected by the particles, and the reflected waves are received by an ultrasonic receiver. The frequency shift and ultrasonic absorption brought about by the particle motion in accordance with the Doppler principle are evaluated. The particles detectable with the device range in size from 1 μm to 1000 μm.
In a cooking apparatus according to German patent document DE 101 43 841 A1, the density of a cooking atmosphere in a cooking chamber is determined by measuring sound velocity, in order to derive therefrom the content of water vapour and thus the humidity of the cooking atmosphere. In this application, account is taken of the fact that the cooking temperature influences the density of the cooking atmosphere. Sound velocity measurement may be performed in a measuring tube, which is connected atmospherically to the cooking chamber. Sound velocity is measured using an ultrasonic transmitter and an ultrasonic receiver, (which may be combined into a single unit), with a known measurement path therebetween.
One object of the present invention is to provide a fuel cell system which includes a device for adjusting the moisture content of the working fluid flowing in one direction in a channel system, and which allows reliable and dynamic measurement of the actual humidity.
This and other objects and advantages are achieved by the fuel cell system according to the invention, which uses an ultrasonic detector to measure the actual humidity. The invention is particularly advantageously applicable to the measurement of the actual humidity on the cathode side of a fuel cell system.
To measure actual humidity, sound velocity is preferably determined in a working fluid flowing in a channel of a channel system, from which a dew point temperature is derived. The ultrasonic detector comprises a transmitter and a receiver, which are arranged at a known distance from one another. At identical temperature, sound velocity in water vapour is higher than in dry air. This means that as the humidity of the working fluid rises, sound velocity increases along the measurement path. Since the vapour pressure of water increases exponentially as the temperature rises, the accuracy of the measurement principle increases on the one hand as the temperature increases, and on the other hand, as the respective dew point of the working fluid is approached.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.